6. The Expansion Tank

When a fluid heats up it expands. The expansion of water heated from 7oC to 100oC is approximately 4%. To avoid the expansion building up a pressure in the system exceeding the design pressure, it is common to led the expanding fluid to a tank - open or or closed.

Open expansion tank

An open expansion tank is only relevant for Low Pressured Hot Water - LPHW - systems. The pressure is limited by the highest location of the tank.

The volume of an open expansion tank should be the double of the estimated expansion volume in the system. The formula below can be used for a hot water system heated from 7oC to 100oC (4%):

Vt = 2 0.04 Vw (8)

where

Vt = volume of expansion tank (m3)

Vw = volume of water in the system (m3)

Closed expansion tank

In an closed expansion tank the pressure in the system is maintained partly by compressed air. The expansion tank volume can be expressed as:

Vt = Ve pw / (pw - pi) (8b)

where

Vt = volume of expansion tank (m3)

Ve = volume by which water contents expands (m3)

pw = absolute pressure of tank at working temperature - operating system (kN/m2)